The invention relates to the treatment of nitrogen-laden natural gas. More particularly, the invention relates to the removal of nitrogen from such natural gas by means of gas-separation membranes.
Fourteen percent of known U.S. natural gas reserves contain more than 4% nitrogen. Many of these reserves cannot be exploited because no economical technology for removing the nitrogen exists.
Cryogenic distillation is the only process that has been used to date on any scale to remove nitrogen from natural gas. The gas streams that have been treated by cryogenic distillation, for example streams from enhanced oil recovery, have large flow rates and high nitrogen concentration, such as more than 10 vol %. Cryogenic plants can be cost-effective in these applications because all the separated products have value. The propane, butane and heavier hydrocarbons can be recovered as natural gas liquids (NGL), the methane/ethane stream can be delivered to the gas pipeline and the nitrogen can be reinjected into the formation.
Cryogenic plants are not used more widely because they are costly and complicated. A particular complication is the need for significant pretreatment to remove water vapor, carbon dioxide and C3+ hydrocarbons and aromatics to avoid freezing of these components in the cryogenic section of the plant, which typically operates at temperatures down to xe2x88x92150xc2x0 C. The degree of pretreatment is of ten far more elaborate and the demands placed upon it are far more stringent than would be required to render the gas acceptable in the pipeline absent the excess nitrogen content. For example, pipeline specification for water vapor is generally about 120 ppm; to be fit to enter a cryogenic plant, the gas must contain no more than 1-2 ppm of water vapor at most. Similarly, 2% carbon dioxide content may pass muster in the pipeline, whereas carbon dioxide must be present at levels no higher than about 100 ppm for cryogenic separation. For streams of flow rates less than about 50-100 MMscfd, therefore, cryogenic technology is simply too expensive and impractical for use.
Other processes that have been considered for performing this separation include pressure swing adsorption and lean oil absorption; none is believed to be in regular industrial use.
Gas separation by means of membranes is known. For example, numerous patents describe membranes and membrane processes for separating oxygen or nitrogen from air, hydrogen from various gas streams and carbon dioxide from natural gas. Such processes are in industrial use, using glassy polymeric membranes. Rubbery polymeric membranes are used to separate volatile organic compounds from air or other gas mixtures.
An application that is very difficult for membranes is the separation of nitrogen from methane. Both glassy and rubbery membranes have very poor selectivities, typically of 3 or less, for nitrogen over methane or methane over nitrogen.
U.S. Pat. No. 3,616,607 to Northern Natural Gas Company, discloses membrane-based separation of nitrogen from methane for natural gas treatment, using nitrogen-selective membranes. The patent reports extraordinarily high nitrogen/methane selectivities up to 15 and 16. These numbers are believed to be erroneous and have not been confirmed elsewhere in the literature. Also, the membranes with these alleged selectivities were made from polyacrylonitrile, a material with extremely low gas permeability of the order 10xe2x88x924 Barrer (ten thousandths of a Barrer) that would be impossible to use for a real process.
It was discovered a few years ago that operating silicone rubber membranes at low temperatures can increase the methane/nitrogen selectivity to as high as 5, 6 or above. U.S. Pat. Nos. 5,669,958 and 5,647,227 make use of this discovery and disclose low-temperature methane/nitrogen separation processes using silicone rubber or similar membranes to preferentially permeate methane and reject nitrogen. However, such a selectivity is obtained only at very low temperatures, typically xe2x88x9260xc2x0 C., for example. Temperatures this low generally cannot be reached by relying on the Joule-Thomson effect to cool the membrane permeate and residue streams, but necessitate additional chilling by means of external refrigeration. While such processes may be workable in industrial facilities with ready access to refrigeration plants, they are impractical in many gas fields, where equipment must be simple, robust and able to function for long periods without operator attention.
Another problem of very low temperature operation is that, even though the membranes themselves may withstand the presence of liquid water and hydrocarbons, considerable pretreatment is of ten necessary to avoid damage to ancillary equipment by condensed liquids. Streams must also be dried to a very low water content to prevent the formation of methane or other hydrocarbon hydrates that can clog the system.
U.S. Pat. Nos. 6,361,582 and 6,361,583, co-owned with the present application, describe processes for separating gases such as nitrogen from C3+ hydrocarbons. The processes make use of membranes made from certain fluorinated polymers that are resistant to plasticization by hydrocarbons. U.S. patent application Ser. No. 10/100,459, entitled xe2x80x9cNitrogen Gas Separation Using Organic-Vapor-Resistant Membranesxe2x80x9d and co-owned and copending with the present application, describes processes for separating nitrogen from gas mixtures such as natural gas using the same type of fluorinated membranes.
U.S. patent application Ser. No. 10/105,861, entitled xe2x80x9cGas Separation Using Organic-Vapor-Resistant Membranes in Conjunction with Organic-Vapor-Selective Membranesxe2x80x9d and co-owned and copending with the present application, describes processes for separating gas mixtures, such as nitrogen-containing natural gas, by means of flow schemes that use combinations of rubbery and glassy membranes.
U.S. Pat. No. 6,425,267, co-owned with the present application, describes processes for removing nitrogen from natural gas using a two-step arrangement of nitrogen-rejecting membranes.
U.S. patent application Ser. No. 10/035,404, entitled xe2x80x9cNatural Gas Separation using Nitrogen-Selective Membranesxe2x80x9d and Ser. No. 10/033,680, entitled xe2x80x9cNatural Gas Separation using Nitrogen-Selective Membranes of Modest Selectivityxe2x80x9d, both co-owned and copending with the present application, describe processes for separating nitrogen from natural gas using only glassy, nitrogen-selective membranes.
There remains a need for improvements to the above-described processes, especially for treating gas streams containing relatively high concentrations of nitrogen, or those where the composition varies over time.
The invention is a process for treating natural gas or other methane-containing gas to remove excess nitrogen.
It is envisaged that the process will be particularly useful as part of a natural gas processing train. Pipeline specification for natural gas is usually no more than about 4% nitrogen, but raw gas frequently contains more than 4% nitrogen and not infrequently contains 10% nitrogen, 20% nitrogen or more. The process of the invention frequently enables gas that is out of specification with respect to nitrogen to be brought to pipeline specification. Gas streams associated with oil wells, landfill gas, coal-seam gas and the like fall within this general type of treatable gas stream.
Other application areas where the process is expected to be useful include, but are not limited to, treatment of off-gases from petrochemical manufacturing and other industrial processes.
The invention relies on membrane separation using a combination of methane-selective membranes and nitrogen-selective membranes. Specifically, the process in preferred form uses three membrane separation units, configured as a two-step, two-stage operation, as defined below. The first and second steps of the first stage use methane-selective membranes, and the second stage uses nitrogen-selective membranes.
In a basic embodiment, the process of the invention includes the following steps:
(a) providing a first membrane unit containing a first membrane having a first feed side and a first permeate side, the first membrane being more permeable to methane than to nitrogen;
(b) providing a second membrane unit containing a second membrane having a second feed side and a second permeate side, the second membrane being more permeable to methane than to nitrogen, the second membrane unit being connected in series with the first membrane unit such that gas leaving the first feed side can enter the second membrane unit on the second feed side;
(c) providing a third membrane unit containing a third membrane having a third feed side and a third permeate side, the third membrane being more permeable to nitrogen than to methane, the third membrane unit being connected in series with the first membrane unit such that gas leaving the first permeate side can enter the third membrane unit on the third feed side;
(c) passing a gas stream, comprising methane and at least about 4% nitrogen, at a first pressure, into the first membrane unit at a first inlet of the first feed side;
(d) withdrawing from a first outlet of the first feed side a first residue stream enriched in nitrogen compared with the gas stream;
(e) withdrawing from the first permeate side, at a second pressure lower than the first pressure, a first permeate stream depleted in nitrogen compared with the gas stream;
(f) passing the first residue stream into the second membrane unit at a second inlet of the second feed side;
(g) withdrawing from a second outlet of the second feed side a second residue stream enriched in nitrogen compared with the first residue stream;
(h) withdrawing from the second permeate side, at a third pressure lower than the first pressure, a second permeate stream depleted in nitrogen compared with the first residue stream;
(i) passing the first permeate stream, at a fourth pressure, into the third membrane unit at a third inlet of the third feed side;
(g) withdrawing from a third outlet of the third feed side a third residue stream depleted in nitrogen compared with the first permeate stream;
(h) withdrawing from the third permeate side, at a fifth pressure lower than the fourth pressure, a third permeate stream enriched in nitrogen compared with the first permeate stream.
The third residue stream is usually the principal product stream of the process, and the process can usually be configured so that this stream meets pipeline specification or other target specification for nitrogen.
Preferred embodiments of the invention include recirculating the third permeate stream to the front of the process to increase methane recovery.
Also preferred, depending on the feed gas composition, is either recirculating the second permeate stream to the front of the process or withdrawing the second permeate stream as a second product stream of value.
If the second and third permeate streams are both fully recirculated, the process produces two product streamsxe2x80x94the third residue stream, with a nitrogen content reduced to the target level, and the nitrogen-rich second residue stream. In this case, this stream may be used in whole or in part as fuel to run field engines, turbines or the like.
If the raw gas stream is very heavily contaminated with nitrogen, such as may occur when treating gases generated by nitrogen injection, the process may be used to produce three product streams by withdrawing, rather than recirculating, a portion or all of the second permeate stream. In this case, the third residue stream, as before, is the methane-rich, nitrogen-depleted product stream, the second permeate stream provides fuel gas, and the second residue stream contains a high concentration of nitrogen, and may be suitable for further use as injection gas.
By adopting one of these preferred embodiments, the fuel to run any compressor needed for the process can be generated as a discrete product stream by the process itself. This is very beneficial as gas-fired compressors can operate in remote locations where an electrical power supply is unavailable.
The process of the invention offers a number of additional features and advantages. Most importantly, it enables natural gas containing relatively large amounts of nitrogen, such as 10%, 20% or higher, to be treated economically to reduce the nitrogen content to a target value, such as 8% nitrogen, 6% nitrogen or less. Frequently, it is possible to bring the stream close to or within pipeline specification of no more than 4% nitrogen. Furthermore, for small gas streams or remote gas fields, these results can be achieved more simply, reliably and cheaply than could be done with prior art technology.
Also, unlike the prior art membrane processes disclosed in U.S. Pat. Nos. 5,669,958 and 5,647,227, it is not necessary to operate the methane-selective membrane separation steps under conditions of such low temperature as to yield a methane/nitrogen selectivity of at least 5. The two-step, two-stage membrane process configuration provides adequate performance, in terms of low product nitrogen content combined with good methane recovery, even when the membrane selectivity is as low as 2, 3 or 4, for example.
The nitrogen-selective materials used for the second stage also have numerically low nitrogen/methane selectivity, typically of only about 2 or 3, becoming more selective for nitrogen over methane as the operating temperature declines. However, again the specific configuration provided herein enables the nitrogen-selective stage to be operated to yield satisfactory results without having to resort to extremely low temperatures.
In general, sufficient cooling to produce adequate selectivity in both stages can, therefore, be provided simply by taking advantage of the cooling by Joule-Thomson effect of both permeate and residue streams that takes place in membrane separation processes.
Such cooling can be accomplished by heat exchange between the membrane feed, residue and permeate streams, and optionally by expanding the membrane residue streams before such heat exchange, without the need for any external refrigeration source. In general, the process can be operated at temperatures above xe2x88x9240xc2x0 C., and of ten much higher, such as above xe2x88x9230xc2x0 C., above xe2x88x9225xc2x0 C., above xe2x88x9210xc2x0 C. or even around 0xc2x0 C. or above. The ability to function at these comparatively high temperatures and without external cooling in many instances is a particular advantage of the present invention, as it greatly simplifies the process compared with prior art technologies.
Furthermore, very high pressures are not needed for good performance. In general, feed gas pressures in the range between about 400 psia and 1,500 psia can be used.
If desired, the process can be operated so as to keep the average temperatures of the membrane separation units and incoming and outgoing streams above about xe2x88x9225xc2x0 C. In this case, metal components of the equipment can be made from carbon steel rather than stainless steel, with considerable cost savings.
The invention is particularly useful for treating gas streams that arise as a result of nitrogen injection processes. Traditional oil-production techniques recover as little as 25-35% of the oil in a typical field. Recovery is improved by injecting carbon dioxide into the reservoir at the periphery. The gas dissolves in the remaining oil and lowers its viscosity, enabling it to be pushed more readily to the extraction wells. High-pressure nitrogen is also injected into gas fields to drive the gas to the wells, as well as to recover methane from coal bed methane reservoirs. The overall economics of such processes are dependent on the costs of the nitrogen injectant, which of ten has to be supplied from a cryogenic plant on site or a similarly costly source. A cost-effective process able to recover nitrogen at a composition suitable for reinjection makes these types of processes more efficient and attractive. The invention can provide such processes.
Many natural gas streams contain excess carbon dioxide. The processes of the invention are also well suited for removing carbon dioxide from the raw gas stream, and may of ten be configured to bring the product gas stream into specification for both carbon dioxide and nitrogen.
In some instances, such as when the raw gas is less heavily contaminated with nitrogen, it may be possible to achieve acceptable results using only a single membrane separation step in the first stage. In this embodiment, the process of the invention includes the following steps:
(a) providing a first membrane unit containing a first membrane having a first feed side and a first permeate side, the first membrane being more permeable to methane than to nitrogen;
(b) providing a second membrane unit containing a second membrane having a second feed side and a second permeate side, the second membrane being more permeable to nitrogen than to methane, the second membrane unit being connected in series with the first membrane unit such that gas leaving the first permeate side can enter the second membrane unit on the second feed side;
(c) passing a gas stream, comprising methane and at least about 4% nitrogen, at a first pressure, into the first membrane unit at a first inlet of the first feed side;
(d) withdrawing from a first outlet of the first feed side a first residue stream enriched in nitrogen compared with the gas stream;
(e) withdrawing from the first permeate side, at a second pressure lower than the first pressure, a first permeate stream depleted in nitrogen compared with the gas stream;
(f) passing the first permeate stream, at a third pressure, into the second membrane unit at a second inlet of the second feed side;
(g) withdrawing from a second outlet of the second feed side a second residue stream depleted in nitrogen compared with the first permeate stream;
(h) withdrawing from the second permeate side, at a fourth pressure lower than the third pressure, a second permeate stream enriched in nitrogen compared with the first permeate stream.
The invention is also particularly useful for treating nitrogen-contaminated natural gas where the composition of the gas changes over time. In many natural gas production processes, the concentration of nitrogen in the raw gas slowly increases. A two-step process using only membranes selective for methane over nitrogen may be able, initially, to produce a permeate from the first membrane separation step that is sufficiently depleted in nitrogen to meet target specifications. As time goes on, however, it becomes increasingly difficult to reduce the permeate nitrogen content to the target level. The use of a nitrogen-selective second stage to polish the gas overcomes this difficulty.
It is an object of the invention to provide a process for removing excess nitrogen from methane/nitrogen gas mixtures.
Other objects and advantages will be apparent from the description of the invention to those skilled in the gas separation arts.